I. Field of the Invention
The present invention relates generally to gas turbine engines and, more particularly, to a triple spool gas turbine engine.
II. Description of the Prior Art
There have been a number of previously known gas turbine engines. Many of these previously known gas turbine engines comprise a housing having an air inlet, an exhaust outlet and a fluid passageway for connecting the inlet to the outlet. A main shaft is rotatably mounted within the housing and has an air compressor means secured to the shaft at one end and a turbine expander means secured to the shaft at its opposite end. A combustor is operatively positioned within the fluid passageway between the compressor and turbine means.
The compressor means for many of these previously known gas turbine engines typically comprise an axial compressor in which the intake air is increasingly compressed prior to its introduction to the combustor. Similarly, the turbine expander means typically comprises a plurality of turbine wheels or stages which progressively increase in size and through which the combustion products or gas stream from the combustor expand to both rotatably drive the compressor and also to provide the thrust for the turbine engine.
For maximum turbine engine efficiency, and hence for minimum fuel comsumption, it is necessary to minimize the internal losses of the engine. Such internal losses result, for example, from gas leakage flow from the high pressure regions of the turbine engine and particularly around the turbine shaft. Such internal losses are particularly disadvantageous for relatively small turbine engines, i.e. turbine engines producing generally less than 10,000 pounds of thrust, since engine efficiency increases with engine size.
Another disadvantage of these previously known gas turbine engines is that the low pressure turbine stages are used to drive the intermediate compressor stages. Since the turbine blade stresses are proportional to the product of the turbine blade annulus area and the speed squared, the low pressure turbine must be designed to withstand the stress levels produced from driving the intermediate compressor stages. Moreover, compromises in the turbine and/or compressor stages is often necessary to enable the various compressor and turbine stages to operate within the acceptable stress levels and such design compromises inherently adversely effect engine efficiency.